A machine’s useful life depends on the life of its critical components. And machine designers frequently hear the question, “How long can I expect this machine to keep working?”

When electric linear actuators are used, calculating life can be straight-forward for ball screw and roller screw actuators. Since these screw types incorporate rolling elements as essential parts, you can use the L10 life formulafor ball bearings.

Distance versus time

The result of an L10 life calculation is in inches or millimeters of travel. This is useful information; however, the individuals asking you how long your machine will last probably want the information in units of time, like days, months or years.

(Of course, L10 is a theoretical calculation of life based on load. As a seasoned designer, you’ll also consider the many other factors that can affect actuator life like the environment and alignment.)

This blog will show you how to convert an L10 result to units of time. But first a quick review.

Important terms

Dynamic Load Rating (DLR): Provided by the manufacturer and represented by the letter C. DLR is a constant load under which a ball bearing device will achieve 1 million revolutions (rotations).

Constant load: A load that doesn’t change during the working cycle.

Varying Load: A load that changes during the working cycle.

Equivalent Dynamic Load: Must be calculated when an application has a varying load. It’s a dynamic load which, if applied constantly, would have the same effect on screw life as the combined actual loads. Represented by the symbol, Pe.

L10 (or B10) Life: L10 life for a group of identical screw actuators operating under the same conditions is the number of revolutions which 90% of these actuators have the statistical probability to achieve (or at which 10% can be expected to fail).